Redirect the standard input of runner user hooks

`run` script invokes user hooks from its `run` function that is called
from inside a `read` block; this way the user scripts share the
input of the `read` command. This can lead to unexpected behaviour
if any of the user scripts include a command that reads from the
standard input.

Redirecting the standard input of user hooks to /dev/null isolates
these scripts from `run` sufficiently.